OF THE SENSITIVE STATE OF VACUUM DISCHARGES. 
589 
the interior surface of the tube at the spot on which the finger rests) phosphorescence 
appeared on the films opposite to the finger. These films were moved by the 
impact of the molecules as in the case of Crookes’ mill; but they were only moved 
very slightly. If, however, the finger was placed close under one of the films it 
was moved readily, showing that though the momentum of the molecules after they 
had crossed the tube was not sufficient to move the film s, yet their initial momentum 
was amply sufficient to do so. The irregular shapes of the films gave opportunities 
of testing in a variety of ways the truth of this conclusion, and in all cases it was 
confirmed. 
The importance of these results is twofold. They not only demonstrate that 
phosphorescence can be obtained at pressures so comparable with ordinary gaseous 
pressures that it is unnecessary, and indeed inadmissible, to have recourse to the 
supposition of an alteration of the ordinary laws of gases ; but they also show that 
these streams of molecules are strongly under the influence of gaseous resistance, and 
that they rapidly lose their velocity from its action, so that, even in cases where phos¬ 
phorescence is not visible, the same molecular streams exist, and may be made to pro¬ 
duce it if proper means are taken to prevent their velocities being checked too much 
by the density of the vapour through which they have to force their way. Thus we 
may fairly conclude that the above-mentioned pressures by no means necessarily 
represent the limit at which these molecular streams exist. If it were desired to 
obtain phosphorescence at still higher pressures, all that would be necessary would 
seem to be to bring the glass intended to be affected into close proximity to the 
place of discharge, and still further to augment the violence of the electric impulses. 
No doubt in this way it would be possible to trace the presence of these molecular 
streams at much higher pressures; and if the thermal instead of the luminous effects 
of their impact on the glass were taken, it is probable that the range of pressures 
might still farther be increased. But it is sufficient for our purpose to show that no 
special condition of gas is necessary for the genesis of these molecular streams, and 
that they enjoy no special exemption from ordinary gaseous action in their subsequent 
path, since our object is not to determine the exact condition under which they occur, 
but to establish the close analogy between the molecular streams that produce phos¬ 
phorescence and the other instances to which we have above referred, in which streams 
of particles are driven off from the negative terminal, and thereby to divest these 
streams of molecules of the character of an unprecedented phenomenon which would 
justify the hypothesis of any considerable change of conditions to account for its 
presence. 
It may perhaps be said that it is unnecessary to give experimental proof that these 
molecular streams are obstructed by the medium through which they pass, so that 
they may actually exist even when no phosphorescence is manifested. This is in one 
sense common to all theories respecting them. The experiments of Crookes with the 
electric radiometer show that the molecular streams seldom penetrate beyond what is 
MDCCCLXXX. 4 G 
